The widespread availability of personal computers at low cost has led to a situation where the public demands access to the Internet and other computer networks at the lowest possible cost. This demand is being expanded to include network access for portable devices such as laptop computers, Personal Digital Assistants, and the like. Users of such portable devices even now expect to be able to access such computer networks with the same convenience that they have grown accustomed to when using wire line connections.
Unfortunately, there is still no widely available satisfactory solution for providing low cost wireless access to the Internet at high speed. At the present time, the users of wireless modems that operate with the existing digital cellular telephone network often experience a difficult time when trying to, for example, view Web pages. The existing digital cellular telephone network may use schemes such as Code Division Multiple Access (CDMA) to permit multiple users to operate on the same Radio Frequency (RF) physical layer channel at the same time, such as in the IS-95B standard which is popular in the United States. In this approach, each traffic signal is first encoded with a pseudorandom (PN) code sequence at the transmitter. The receivers include equipment to perform a PN decoding function in such a way that signals encoded with different PN code sequences or with different code phases can be separated from one another. Because PN codes in and of themselves do not provide perfect separation of the channels, these systems have an additional layer of coding, referred to as “orthogonal codes.” The orthogonal codes further reduce interference between channels.
However, the higher layer communication protocols in such networks were originally designed to support voice communication and not the packet-oriented data communication protocols used for connecting users of wide area networks do not lend themselves to efficient transmission over wireless interfaces.
Certain other CDMA systems have been proposed that more efficiently support data communications using multiple connections over a given Radio Frequency (RF) channel. One example of such a system was described in U.S. Pat. No. 6,151,332 entitled “A Protocol Conversion and Bandwidth Reduction Technique Providing Multiple nB+D ISDN Basic Rate Interface Links Over a Wireless Code Division Multiple Access Communication System,” and assigned to Tantivy Communications, Inc., the assignee of the present application. With such techniques, higher speed throughput can be provided by a more efficient allocation of access to the CDMA channels. In particular, a number of sub-channels are defined within a standard CDMA channel bandwidth, such as by assigning a different code to each sub-channel. The instantaneous bandwidth needs of a given connection are then met by dynamically allocating multiple sub-channels on an as-needed basis for each session. For example, sub-channels can be granted during times when the subscriber bandwidth requirements are relatively high, such as when downloading Web pages. The sub-channels are then released during times when the content is relatively light, such as when the user is reading a previously downloaded Web page.
However, to implement such a system requires careful planning of various modulation and coding schemes in order to accomplish the maximum possible bit rate while minimizing the effects of noise, multi-path distortion, and other sources of errors. For example, modulation codes and pseudorandom spreading codes must be carefully selected to minimize interference among channels occupying the same radio frequency carrier. In addition, it is necessary for framing bits to be inserted in data streams so that higher layered data protocols such as Transmission Control Protocol/Internet Protocol (TCP/IP) can be efficiently implemented.
Furthermore, in order for the PN and orthogonal code properties to operate properly at a receiver, certain other design considerations must be taken into account. For signals traveling in a reverse link direction, that is, from a mobile unit back to a central base station, power levels must be carefully controlled. In particular, the orthogonal properties of the codes are optimized for the situation where individual signals arrive at the receiver with approximately the same power level. If they do not, channel interference increases.
The forward link direction presents a different problem. A signal traveling from the base station to a subscriber unit may interfere with another signal in an unpredictable way as a result of the so-called near-far problem. For example, far away mobile units require relatively high power in order to be detected properly whereas close-in mobile units require lower power. The stronger signals may interfere with proper operation of mobile units located closer to the base station which typically to operate with lower power levels. Unfortunately, this behavior depends upon the specific operating environment of the mobile communications system, including the topology of the surrounding geography, the juxtaposition of the subscriber units with respect to one another, and other factors.
In the past, with voice-based systems such as IS-95, it has been possible to set power levels individually to optimize each forward link channel so that interference is minimized. With these systems, since the information bandwidth remains constant, a transmitted power level can be continuously adjusted in a closed-loop fashion to affect an optimum received power level at the subscriber unit which tends to minimize interference.